Studies of the occurrence and properties of membrane bound enzymes of liver and kidney which catalyze the synthesis and hydrolysis of a variety of phosphorylated sugars and sugar alcohols have been continued. An enzyme has been found in Limulus polyphemus which resembles vertebrate glucose-6-phosphatase in its specific anatomical distribution, pH optimum, kinetic properties, phosphotransferase activity, substrate specificity and phospholipid dependence. A variety of other invertebrates tested exhibited little or no enzyme with these properties. The hypothesis that a specific glucose-6-phosphatase is to be found only in those animals which utilize free glucose as an important circulating form of energy has been proposed. It appears that a variety of transport compounds, such as trehalose and glucose was tried at the evolutionary level of the Arthropods. The development of glucose-6-phosphatase and related enzyme activities in the liver and kidneys of neonatal rats has been studied as a function of age.